Hydrated zinc borate, dimethyl silicone resin arc extinguishing material

ABSTRACT

Arc-extinguishing materials are selected in accordance with criteria such as first ionization potential of each of the constituents, carbon content, boiling, sublimation, or decomposition temperature, endothermic character of decomposition, electronegativity of decomposition products, rate of reformation, dimensional and chemical stability, ease of application, cost, toxicity, and coefficient of thermal conductivity.

United States Patent 1 [111 3,912,650 Khalid et a1. Oct. 14, 1975 .[54]HYDRATED ZINC BORATE, DINIETHYL 3,328,481 6/ 1967 Vincent 260/37 SBSILICONE RESIN ARC EXTINGUISHING 3,525,707 8/ 1970 Bobear 260/37 SBMATERIAL [75] Inventors: Joseph M. Khalid; Richard W. Primary Examl'ilerBen-lamfn Padgett Niccons, both of Cedar Rapids Iowa AssistantExammer-losephme Lloyd Attorney, Agent, or Fzrm-Harold J. Rathbun;Ernest [73] Assignee: Square D Company, Park Ridge, 111. S Kettelson[22] Filed: May 29, 1973 [21] Appl. No.: 364,597 [57] ABSTRACTArc-extinguishing materials are selected in accordance Cl 200/144 withcriteria such as first ionization potential of each 37 252/637 of theconstituents, carbon content, boiling, sublima- [51] Int. Cl. H01B 3/46tion or decompgsition temperature, endothermic Field of Search characterof decomposition, electronegativity of de- 260/37 200/144 C compositionproducts, rate of reformation, dimensional and chemical stability, easeof application, cost, [56] R f r s Cited toxicity, and coefficient ofthermal conductivity.

5 Claims, N0 Drawings IIYDRATED ZINC BORATE, DIMETI-IYL SILICONE RESINARC EXTINGUISIIING MATERIAL This invention relates to arc-extinguishingmaterials usable as molding compositions for molding arc shields, arcchute frame components, cases of electrical devices, and the like andalso usable in coating form for coating surface portions of a memberwholly' or partly defining an arc chamber in which separablecontacts ofan electrical circuit interrupting device are disposed.

Arc interruption requires the'establishment of conditions promoting ahigher rate of recombination than the rate of ionization. The problemreduces to one of recognizing the basic factors that control these tworates.

One of the factors which should be considered in the selection ofarc-extinguishingimaterials is the first ionization potential of each ofthe constituents of the material The first ionization potential of anelement is the amount of energy required'to remove one electron away toinfinity from a neutral atom of that element. For silver, the firstionization potential is 7.54 electron volts, while the firstioniz ationpotential for aluminum is 5.98 electron volts. To appreciate thesignificance of a difference of one electron voltbetween the firstionization potentials of two elem ent s, consider that the temperaturedifference between two particles having an energy difference ofone'electroh volt'is 7,730 Centigrade.

Another factor which should be considered in the selection ofarc-extinguishing materials is electronegativity of decompositionproducts. This is a measure of the affinity of an atom of an element forelectrons. As an arc current approaches zero it would be desirable tohave an electronegative gas in the arc chamber that would capture thecooling and slowing electrons. Such electrons would otherwise beaccelerated by the rising recovery voltage and cause further ionizationand possible reignition.

Yet another factor which should be considered in the selection ofarc-extinguishing materials is the boiling sublimation, or decompositiontemperature. During the few microseconds preceding and following thepassing of the current through zero in an alternating current system,the arc column temperature decreases rapidly until it reaches thetemperature of the arc chamber walls, which act as a constanttemperature sink. The higher the boiling temperature of the material ofthe arc chamber walls is, the lower the recombination rates will be.

Endothermic processes have their primary importance at current zero,particularly in the mode of arc interruption wherein a post-arc currentflows. Under this latter condition the energy being absorbed in anendothermic process taking place in the arc chamber on a modest scalemay approximate the energy being put into the chamber by the post-arccurrent. This effect, complemented by a favorable boiling temperature,significantly contributes to.the success of arc extinction.

If an arc-extinguishing material decomposes to yield free carbon, thiscarbon may be oxidized to carbon dioxide. The process is exothermic, andits timing is bad, because it is as the arc current approaches zero andthe temperature drops and recombination of ions and dissociatedmolecules starts to take place that the exothermic formation of carbondioxide occurs. Deposits of free carboncan'also cause tracking anddielectric failure. However, to limit arc-extinguishing materials tothose containing no carbon would too severely limit the choice ofmaterials. Moreover, hydrogen has been thermic on formation. Thereforeit is desirable to select materials having formulas such that theprobability of reformation after decomposition is small. Aluminum oxide(M 0 and boron oxide (B 0 are materials with such formulas, because theprobability of two aluminum or boron atoms'colliding simultaneously withthree oxygen atoms, or with one molecule and one atom of oxygen, issmall. However, aluminum oxide is I undesirable as an arc-extinguishingmaterial because the ionization potential of aluminum is low and theboiling temperature of aluminum oxide is too high. In this respect, thepresent invention is a departure from the prior art as represented byU.S. Pat. Nos. 2,768,264, and 3,071,666 which tout aluminum oxide as agood arcextinguishing material.

SUMMARY OF CRITERIA In accordance with the invention, the firstionization potential of each of the constituents of a suitablearcextinguishing material, should be equal to or greater than 7.54electron volts, which is the first ionization potential of silver. Theatomic concentration of carbon in the material should not be greaterthan about fifteen per cent. The boiling, sublimation, or decompositiontemperature of the material should be as low as possible consistent withother requirements, preferably below two thousand degrees Centigrade.Decomposition of the material should be a strongly endothermic process,the more endothermic the better. The products of decomposition of thematerial should be as electronegative as possible consistent with otherrequirements. The material should have a formula such that the rate ofreformation after decomposition is vanishingly small near the zero pointof the alternating current, such as that for B 0 or H The materialshould be non-toxic, non-caustic, non-inflammable, easy to process andapply, and dimensionally and chemically stable under operatingconditions. Further, it should have good thermal conductivity for anelectrically insulating material, preferably greater than 0.001 caloriesper second per square centimeter per degree Centigrade per centimeter.

REPRESENTATIVE MATERIALS In general the oxides, borides, borates,silicates, and the ammonium complexes of the elements having a firstionization potential equal to or greater than that of silver (7.54eV)are suitable arc-extinguishing materials, and so are their hydratedforms. A preferred selection of such elements, with their firstionization potentials shown in parentheses, is tantalum (7.70eV), copper(7.72eV), cobalt (7.86eV), rhenium (7.87eV), iron (7.90eV), tungsten(7.98eV), silicon (8.l5eV), boron (8.29eV), palladium (8.30eV), antimony(8.64eV),

tellurium (9.0leV), zinc (9.39), and selenium (9.75eV). These substancesmay also be used as fillers in suitable resins provided that the atomiccarbon concentration does not exceed the nominal limit of percent.Silicone resins are suitable. The use of resins becomes mandatory whenthe filler material cannot be used in its pure form. For example, puresilicon dioxide cannot be used because its boiling temperature is toohigh, and pure boric acid cannot be used because it is water soluble andtherefore dimensionally unstable, but these materials are suitable whenused in suitable resins.

Beryllium (9.32eV), arsenic (9.8leV), and mercury (10.43eV) have firstionization potentials higher than that of silver, but these elements aretoxic and therefore undesirable. Sulphates, phosphates, nitrates, andhalogens are also undesirable as toxic.

Silver, platinum. and gold oxides, borates and silicates, with orwithout ammonium complexes, would be suitable arc-extinguishingmaterials except for high cost.

In particular, a material composed of 55.8 per cent by weight FirebrakeZB and 44.2 per cent Sylgard 182 has been found to be a very suitablearc-extinguishing material. Firebrake ZB is made by United States Boraxand Chemical Corporation of Los Angeles, Calif, and has a formula2ZnO.3B O .3%H O. Sylgard 182 is made by Dow Corning Corporation ofMidland, Mich. and is a dimethyl silicone resin having a basic monomerrepresented by the following structural formula:

SiO

3. An arc-extinguishing material as claimed in claim 1 wherein thefiller comprises at least 40 percent by weight of the material.

4. An arc-extinguishing material as claimed in claim 1 wherein thefiller comprises at least 55 percent by weight of the material.

5. An arc-extinguishing material as claimed in claim 1 wherein thefiller comprises about 56 percent by weight of the material.

1. AN ARC-EXTENGUISHING MATERIAL COMPRISNG A FILLER OF HYDRATED ZINCBORATE IN A DIMETHYL SILICONE RESIN, THE CARBON ATOMS IN SAID RESINBEING NO GREATER THAN 15 PERCENT OF THE TOTAL NUMBER OF ATOMS IN SAIDHYDRATED ZINC BORATE AND DIMETHYL SILICONE COMBINED.
 2. Anarc-extinguishing material as claimed in claim 1 wherein the filler hasthe formula 2ZnO.3B2O3.3 1/2 H2O and the resin has a basic monomerrepresented by the structural formula
 3. An arc-extinguishing materialas claimed in claim 1 wherein the filler comprises at least 40 percentby weight of the material.
 4. An arc-extinguishing material as claimedin claim 1 wherein the filler comprises at least 55 percent by weight ofthe material.
 5. An arc-extinguishing material as claimed in claim 1wherein the filler comprises about 56 percent by weight of the material.